Characterizing water treatment residuals phosphorus retention

Research conducted elsewhere has shown that P movement from P-enriched soils to freshwater supplies is reduced when the soils are treated with water treatment residuals (WTRs). The primary purpose of this study was to investigate the ability of three WTRs produced in Florida to reduce soluble P levels in Florida soils amended with fertilizer, manure, and biosolids-P sources. Laboratory studies confirmed that all three WTRs could reduce P solubility. Phosphorus was immobilized by adsorption, precipitation, or both mechanisms. The Al-WTR had an especially large P-sorption capacity of at least 5000 mg P kg^-1. Sorption of P by Al-WTR and Fe-WTR was independent of pH, and essentially irreversible. Equilibrium soluble Al concentrations in the Al-WTR systems were usually <1 mg L^-1 (and decreased further as Al-WTR rates increased), suggesting minimal hazard of Al phytotoxicity to plants. The combined data suggest that WTRs can be effective amendments to improve P retention in poorly P-sorbing Florida soils when applied at practical rates of several Mg ha^-1. Further laboratory studies of the long-term stability of the immobilized P, however, are needed to confirm this work.